Radio direction finding is a method of determining from received radio signals the direction in which the radio transmitter lies. A common method of performing radio direction finding is to configure an array of antennas to receive the signals and determine the direction of the transmitter using the differences in phase and amplitude in the signals received at each antenna in the array. As mobile telephones, as well as other mobile devices, move towards becoming ubiquitous devices there is a desire to provide radio direction finding capability within such devices. It is already very common for such mobile devices to include a wireless radio transceiver, for example, a WiFi transceiver or Bluetooth transceiver, which allows relatively short-range data communication to occur between similarly enabled devices. Consequently, there is a desire to use the existing radio transceivers, such as Bluetooth, to perform radio direction finding using such mobile devices. However, it is expensive in terms of cost and physical space to provide a mobile device having a plurality of radio antennas to perform the radio direction finding method mentioned above.
To overcome this problem the present applicants have already developed a method of radio direction finding that makes use of the particular characteristic of the data packets transmitted using the Bluetooth transmission protocol. In this method, described in the applicants co-pending UK patent application No. 0816688.6 (and counterpart U.S. Patent Application Publication No. 2010/0075603), use is made of the fact that within each header packet of a Bluetooth transmission each data symbol is transmitted as three repeated bits. This allows the radio signal received at two separate antennas within a single mobile device to be sampled by switching between the two separate antennas within the time period over which the three bits are transmitted. This allows phase variations between different samples to be eliminated such that relative phase data for the different antennas can be derived, which can then be subsequently used to perform conventional radio direction finding algorithms.
However, whilst the method described in UK patent application No. 0816688.6 allows radio direction finding to be performed using a single data packet, from a user operability and quality of user experience point of view it is more desirable for the radio direction finding process to be performed on a stream of data packets to allow a pseudo real time direction feedback to be provided to a user. Additionally, in the method described in UK patent application No. 0816688.6, and also in other radio direction finding techniques, actual data content of the received packet is lost, or at best rendered unverifiable. To ensure that existing wireless devices that do not have dedicated radio direction finding capabilities incorporated within them are nonetheless locatable using radio direction finding techniques it is more desirable to be able to perform the radio direction technique by using any data packets transmitted during the normal operation of such devices and without interfering with the transmission of those packets to such an extent that information being conveyed by those packets is lost.